Hydrogen sulfide (H.sub.2 S) and methyl mercaptan (CH.sub.3 SH) are well-known to be the principal contributors to oral malodor. Both compounds arise from degradation of various substances in the mouth through the action of oral microorganisms. The quantity of hydrogen sulfide present in the human breath is typically about three times that of methyl mercaptan.
Hydrogen sulfide in gas streams or the atmosphere may be determined by a number of scientific methods of measurement. The standard method for determining the concentration of H.sub.2 S in the ambient atmosphere involves passing a sample through a suspension of cadmium hydroxide, Cd(OH).sub.2, followed by reaction with P-amino-N,N-dimethylaniline to form methylene blue. The most widely used apparatus for detection of H.sub.2 S operates by passing the gas stream through a paper tape impregnated with lead acetate, mercuric chloride, silver nitrate or potassium dicyanoargentate with the concentration of H.sub.2 S being measured as a function of the optical density of the resulting metal sulfide precipitate. Other methods, including titration methods, gas chromatography, chemiluminescence, and semi-conductor surface alteration may be used. However, all of these methods are cumbersome, slow or demanding in other respects.
To achieve the two principal goals of small size and fast response, an electrochemical method offers the only practical alternative. Such a method is based on the oxidation of H.sub.2 S at the anode and the simultaneous reduction of hydrogen at the cathode to yield a flux of electrons through a closed circuit from anode to cathode. The electrode reactions are as follows: EQU Anode: H.sub.2 S+H.sub.2 O.fwdarw.H.sub.2 SO.sub.4 +8H.sup.+ +8e.sup.-( 1) EQU Cathode: O.sub.2 +4H.sup.+ +4e.sup.- .fwdarw.2H.sub.2 O (2)
These reactions may be catalyzed by use of electrodes which are constructed of catalytic materials, such as, noble metals. The combination of reactions (1) and (2) yields the overall reaction: H.sub.2 S+2O.sub.2 .fwdarw.H.sub.2 SO.sub.4. These reactions are intrinsically rapid when they are properly catalyzed, but they can proceed only as rapidly as the material to be oxidized (H.sub.2 S) reaches the anode.
Previous devices based on electrochemical methods have employed so-called diffusion electrodes, which typically depend on the use of a permeable film of polymeric material, e.g., TEFLON.RTM., for the purpose of retaining the liquid electrolyte solution while permitting the diffusion of gas through the permeable film. Use of such a film is intended to retard loss of water or other electrolytic solvent by evaporation. However, the film also retards the rate at which the gas is able to reach the electrode. The result is a retardation of the response time of the device as a whole.
The purpose of the permeable film in previous inventions is to regulate the concentration of the electrolyte solution such that this concentration remains essentially constant. In particular, the loss of water due to migration of moisture in and out of the electrolyte solution is suppressed by use of a gas-permeable, hydrophobic membrane for the permeable film. Such a membrane has a pore structure which permits the diffusion of gas inward into the electrolyte solution, but its hydrophobic nature supresses the migration of water outward from the electrolyte solution.
One measure proposed to prevent the loss of water from the electrolyte solution is the use of humectant substances dissolved in the electrolyte solution. Another measure proposed is the use of a solid electrolyte instead of a liquid. Such a solid electrolyte may be a completely solid matrix or a rigid aqueous gel. Still another measure to compensate for the tendency of an aqueous electrolyte to lose or gain water and thus change its composition is the use of buffering chemicals in the electrolyte solution. In addition, this measure will compensate for any alteration in an electrolyte solution pH due to variation of the strength of the electrolyte.
U.S. Pat. No. 4,169,779 to Tataria et al describes an electrochemical cell for the detection of hydrogen sulfide. This cell is constructed with the explicit intention of avoiding the use of an aqueous electrolyte because of the sensitivity of such an electrolyte to the humidity in the air. Instead of an aqueous electrolyte, the cell employs an electrolyte which consists of lithium perchlorate or other inorganic salt dissolved in an organic solvent. The cell has a low detectability level for hydrogen sulfide, but has a very slow response time (e.g., up to six minutes to achieve 90% of maximum reading).
The main disadvantages of the gas sensors described in the prior art, namely, slow response and limited shelf life, are overcome in accordance with the present invention by maintaining the solution in a sealed condition when not in use so as to avoid the need for a permeable film which shields the electrolyte solution from direct contact with the gas. Specifically, in the present invention the gas dissolves in the aqueous electrolyte solution in the immediate vicinity of the electrode without the interference of an intermediate film. This feature greatly accelerates the rate at which the gases at the respective electrodes can enter into the reactions described above to within a matter of seconds. At the same time, it is desirable to provide an extremely compact, lightweight container which is normally sealed to prevent loss of the solution through evaporation and can be stored in a pocket or purse so as to be readily available for use in detecting malodor in the human breath.